The present invention relates to the improvements of a variable compression ratio mechanism for a reciprocating internal combustion engine.
In recent years, there have been proposed and developed various variable compression ratio mechanisms for reciprocating internal combustion engines. One such variable compression ratio mechanism has been disclosed in Japanese Patent Provisional Publication No. 9-228858 (hereinafter is referred to as JP9-228858). JP9-228858 teaches the use of an oscillating or rockable lever (called a bridge) provided between a control arm (called a rocking arm) and a connecting rod, for the purpose of varying the position of top dead center of a piston by oscillating motion of the so-called bridge, thereby varying the compression ratio. In the reciprocating engine with such a variable compression ratio mechanism, the piston stroke is 2 times or more the radius of a crank, in accordance with the principle of lever-and-fulcrum or leverage. In comparison with a radius of a crank of a typical reciprocating internal combustion engine with a piston crank mechanism and of the same engine""s displacement, the crank radius of the reciprocating engine with the variable compression ratio mechanism can be reduced or shortened. This enables increased overlap between a crankpin and a crankshaft main-bearing journal, thus enhancing the rigidity of the crank. Therefore, the reciprocating engine with the variable compression ratio mechanism carries the advantage of increasing the mechanical strength of the crank, and of attenuating noise and vibration during operation of the engine.
However, in the reciprocating engine disclosed in JP9-228858, the crankpin is located on a perpendicular line at substantially the midpoint of the bridge, and additionally the lower end of the connecting rod and the lower end of the rocking arm are rotatably linked respectively to both ends of the bridge by way of a pin-connection. Consider an input force Fp acting on the crankpin, an input force Fp1 acting on a first connecting pin via which the connecting rod and the bridge are linked to each other, and an input force Fp2 acting on a second connecting pin via which the bridge and the rocking arm are linked to each other. Assuming that the moments of the forces Fp1 and Fp2 about the crankpin are balanced and the crankpin is located just at the central portion of the bridge, the magnitude of force Fp1 is equal to the magnitude of force Fp2 (Fp1=Fp2), because the distance between the first connecting pin and the center of the bridge is identical to the distance between the second connecting pin and the center of the bridge. As viewed from equilibrium of forces, the summation (Fp1+Fp2) of the two forces Fp1 and Fp2 acting on the respective connecting pins is equivalent to the force Fp acting on the crankpin, that is, Fp=Fp1+Fp2=2Fp1. In other words, two times input load applied to the piston is input into the crankpin journal portion and/or bearing inserts fitted to the central bore of the bridge. To provide the same resistance and durability against the same bearing pressure, the bearing surface area must be increased or the resistance against bearing pressure must be increased. There are some demerits, that is, reduced wear resistance, increased production costs, friction loss, and the like.
Accordingly, it is an object of the invention to provide a variable compression ratio mechanism for a reciprocating internal combustion engine, which avoids the aforementioned disadvantages.
It is another object of the invention to provide a variable compression ratio mechanism for a reciprocating internal combustion engine which is capable of balancing two contradictory requirements, that is, increased piston stroke and reduced load applied to a crankpin.
In order to accomplish the aforementioned and other objects of the present invention, a variable compression ratio mechanism for a reciprocating internal combustion engine comprises a connecting rod connecting a crank on a crankshaft with a piston, the connecting rod being split into an upper connecting rod portion oscillatingly linked to the piston through a piston pin and a lower connecting rod portion rotatably linked to a crankpin of the crankshaft, the upper and lower connecting rod portions being oscillatingly linked to each other through a first connecting pin, a rockable arm oscillatingly linked at one end to the lower connecting rod portion through a second connecting pin, a control mechanism shifting a center of oscillating motion of the rockable arm to vary a compression ratio of the engine, the rockable arm being oscillatingly linked at its other end via the control mechanism to a cylinder block, a piston stroke of the piston being set to be greater than two times a crank radius of the crank on the crankshaft, irrespective of whether the compression ratio is varied by the control mechanism, and a linkage having at least the upper and lower connecting rod portions, the first and second connecting pins and the rockable arm being dimensioned and laid out, so that a crankpin load acting on the crankpin is less than a crankpin load produced by a linkage that has the crankpin located on a perpendicular line at substantially a midpoint of a line segment between and including a center of the first connecting pin and a center of the second connecting pin.
According to another aspect of the invention, a variable compression ratio mechanism for a reciprocating internal combustion engine comprises a connecting rod connecting a crank on a crankshaft with a piston, the connecting rod being split into an upper connecting rod portion oscillatingly linked to the piston through a piston pin and a lower connecting rod portion rotatably linked to a crankpin of the crankshaft, the upper and lower connecting rod portions being oscillatingly linked to each other through a first connecting pin, a rockable arm oscillatingly linked at one end to the lower connecting rod portion through a second connecting pin, a compression-ratio control means for shifting a center of oscillating motion of the rockable arm to vary a compression ratio of the engine, the rockable arm being oscillatingly linked at its other end via the compression-ratio control means to a cylinder block, a piston stroke of the piston being set to be greater than two times a crank radius of the crank on the crankshaft, irrespective of whether the compression ratio is varied by the compression-ratio control means, and a linkage having at least the upper and lower connecting rod portions, the first and second connecting pins and the rockable arm being dimensioned and laid out, so that an arm length for a moment of a force acting on the first connecting pin about the crankpin is shortened relatively to an arm length for a moment of a force acting on the second connecting pin about the crankpin.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.